Fixed Debug::dump() when called from call_user_func_array()

I made shortcut function like below.
```php
function d()
{
	$args = func_get_args();
	call_user_func_array('\\Debug::dump', $args);
}
```
But then, I got error.
```html
ErrorException [ Notice ]: Undefined index: file

COREPATH/classes/debug.php @ line 57
```
```php
// If being called from within, show the file above in the backtrack
 if (strpos($backtrace[0]['file'], 'core/classes/debug.php') !== FALSE)
```

Is this a bug or not?
If this is a bug, I attach a little draft fix.
Can you check it?